Invasive H. influenzae type b disease is a serious health problem of infants and children. One virulence factor possessed by this bacterium is capsular polysaccharide. Age-dependent "natural" immunity to such infections is associated with the presence of serum antibodies directed to the capsule. Unfortunately, the polysaccharide is a poor immunogen in children less than 2 years old; and age group in which 75% of the cases occur. Surface fimbriae are an accepted virulence factor in enterobacteraceae; such structures facilitate adherence, tissue invasion and can be important in the transmission of antibiotic resistance (pili). Fibriate structures have been recently recognized on invasive H. influenzae, and convalescent sera contained antibodies to these surface proteins. However, fimbriae are not detectable on fresh blood or CSF isolates. This proposal seeks to define the role of the surface fibrils in the pathogenesis of invasive H. influenzae type b disease. To unambiquously define the role of these structures, we intend to clone the gene which encodes for their expression from these prototypic type b isolates. Using the cloned gene the contribution of the fimbriae to virulence (and role in acquisition of antibiotic resistance) can be defined by step-wise construction of an invasive H. influenzae in vitro. The regulation of fimbrial gene expression in the wild prototypes and reconstructed strains will be examined. The cloned fimbrial genome will be a potent tool permitting certain chemical characterizations of the protein fimbriae, their interrelationship, and purification of the structure. The immunogenicity and antigenic relationship of the fimbriae from these three strains will be examined. If these structures prove important in tissue invasion by type b H. influenzae, then the availability of the cloned gene will permit quantitative production of a pure vaccinogen.